I am designing a low impact house. I had planned to use a reflecting pool of water to increase the passive solar gain in the house in winter: enter image description here

and reflect it away in the summer (note the structure on the left side of the house is a lean to greenhouse which is why the light is shown as traveling through it): enter image description here

Someone reading my plans suggested that:

Whilst the reflectance pool would undoubtedly contribute to daylighting and would help in the winter months to reduce use of artificial lighting, it is suggested that the water may absorb most of the incoming shortwave energy and release longwave radiation (heat) back to the sky.

Is above statement true?

A StackExchange on whether reflection preserve wavelength suggests that a perfect mirror will preserve wavelength. Reflecting both heat and light. I understand that the surface of the water is not a perfect mirror and that only a proportion of the light/heat would be reflected.

This meteorologist seems to confirm that a low sun will reflect heat:

When the sun is low on the horizon (low sun angle) more radiation will be reflected off water.. A low sun angle has a more difficult time warming water because a great percentage of the solar radiation is reflected away.

Would the reflecting pool illustrated contribute heat to the house in the winter? Would the sunlight reflected by it contain longwave radiation (heat)?

  • $\begingroup$ Your phrasing seems to suggest you establish a correlation between the season and the height of the sun in the sky? low winter sun/high summer sun. Also, a quick look at the wikipedia page for albedo seems to bring most of the info you need to work out an answer. $\endgroup$ Sep 8, 2021 at 15:07
  • $\begingroup$ While on a sailing trip I applied suncream to my nose and cheeks but got sunburn under my chin. $\endgroup$ Sep 8, 2021 at 15:17
  • $\begingroup$ Re, "both heat and light." Solar radiation is 100% light. Some of it (about half) is invisible light, but that is not what "heat" means. "Heat" means something else. Compare en.wikipedia.org/wiki/Heat and en.wikipedia.org/wiki/Infrared $\endgroup$ Sep 8, 2021 at 18:16
  • $\begingroup$ @BarbaudJulien - thank you for your comments. The Albedo wikipedia page is useful thanks. Yes, I've tried to consider the maximum sun angle for the time of year. I have illustrated: Winter sun angle: 51.5 degrees latitude + 23.5 (tilt of earth) = 75 90- 74.5 = 15 degree angle of elevation from the horizon of the sun at midday on the winter solstice. Summer sun angle: 51.5 degrees latitude - 23.5 (tilt of earth) = 28 90- 28 = 62 degree angle of elevation from the horizon of the sun at midday on the summer solstice. $\endgroup$ Sep 9, 2021 at 9:05

1 Answer 1


In short, yes. According to the Fresnel equations, reflection is higher at low angles. https://en.wikipedia.org/wiki/Fresnel_equations

It is true that water absorbs shortwave radiation but that's only if the radiation is passing through the water. Ultimately, Maxwell's equations have to be satisfied at the interface, which causes the grazing reflection that you have likely noticed in your everyday experiences with water surfaces.

Comparing the water scenario to having a patch of ground instead, the water is likely better for heating the house. But since it is most effective at low angles, it is unclear whether the Sun can be low enough to cause significant heating. It is also unclear whether much power will be transmitted, because the effective surface area of the pool is reduced: $A_{\rm eff} = A_{\rm pool} \sin{\theta}$ when the Sun's angle is low. Not clear whether the higher-impact inclusion of the pool ends up balancing the lower-impact heating.


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